The systemic study of aluminium speciation in medium concentrated aqueous solutions

Industrial applications and environmental problems involving the aqueous chemistry of aluminium require an understanding of the speciation of this metal ion at a wide range of concentrations. The formation of polynuclear species is of special interest due to the complexity of the hydrolysis mechanisms and the diversity of the hydrolysis products. Kinetic aspects of speciation are also important considering the different stability ranges of polycationic species formed during the hydrolysis process. In the present paper we report results of systematic studies on the formation of aluminium polycations at room temperature. Automated potentiometric titrations have been used to study the hydrolysis of aluminium-ions in solutions (0.01-0.2 M) on a short time scale (2 min between titrant additions). 27Al NMR spectroscopy and dynamic light scattering have been used for investigations on a longer time scale (24 h). The effects of alkali strength (KOH, NH4OH and KHCO3) and concentration (0.45-2.0 M), counterion identity (Cl(-), NO3(-), SO4(2-)) and ionic strength have been investigated. Optimum conditions for the generation of Al13-mer are proposed on short and long time scales. On a short time scale, aluminium chloride and nitrate should be used as starting materials, KOH and KHCO3 should be used for hydrolysis and experiments conducted at low ionic strength. For solutions that have been left to age, there is a considerable hydrolysis window that can be used to generate significant quantities of the Al13-mer that vary little with the alkali used. Aluminium tridecameric (Al(13)-mer) species are not generated from alum as the precursor. The presence of sulphate ions alters the pathway of aluminium polymerisation to form polymeric and solid materials. On the basis of the potentiometric titration data, dynamic light scattering and 27Al NMR measurements evidence is provided for the detrimental role of sulphate-ions in the formation of Al(13)-mer and an alternative mechanism of aluminium ion polycondensation is proposed, based on the increased stability of monomeric and oligomeric species (dimer and trimer) in the presence of sulphate-ions.